UNIT 2 BRAKES
A Brake is defined as a mechanical device, which is used to absorb the
energy possessed by a moving device, system or mechanism by means
of friction.
The primary purpose of the brake is to slow down or completely
stop the motion of a moving system, such a rotating drum, machine or a
vehicle.

Brakes
The energy absorbed by brake is converted into heat energy
and dissipated to surrounding i.e.
Heat Dissipation is a serious problem in brake applications.
Brakes are classified into following 3 groups
1. Mechanical brakes – operated by means of
levers, springs and pedals
2. Hydraulic and Pneumatic brakes – operated by fluid
pressure such as oil pressure or air pressure
3. Electrical Brakes – Operated by eletro-magnetic forces.`
Types of Mechanical brakes: Shoe brakes, Band brakes,
Internal and External expanding brakes

Energy Equations
The first step in design of a mechanical brake is to determine the
braking torque capacity . The braking torque capacity depends
upon the amount of energy to be absorbed by the brake.

Chapter 12

Chapter 12

Chapter 12

Chapter 12

Chapter 12

Numerical Problems

Chapter 12

Chapter 12

Block Brake with short shoe
A block brake consists of a simple block,
which is pressed against the rotating
drum by means of a lever as shown in
figure. The friction between the block
and brake drum causes the retardation of
drum.
Application: On rail wheels.
Angle of contact between block and
brake is usually small < 45O. Since
angle of contact is small, it results in
uniform pressure distribution on block.

Free Body Diagram (Clockwise Rotation)
Mt – Braking torque (N-mm)
R – radius of brake drum (mm)
μ - coff. of friction
N – Normal reaction (N)
The dimensions of block are determined by
N= plw
Here p – permissible pressure between block
and brake drum (N/mm2)
l= length of block
w = width of block
A Narrow Block (lesser w)
drawback-> Size – large
A Broad Block (Higher w)
drawback -> Pressure distribution is
non-uniform

Free Body Diagram (Clockwise Rotation)
Assumptions
The width of block should be optimum between
2 limits given by:
(1/4 )* drum diameter < w < (1/2)*
drum diameter

Free Body Diagram (Clockwise Rotation)
Considering the equilibrium of forces in vertical and
horizontal directions:
Taking moments of forces acting on lever about the
hinge point O,
Depending upon magnitude of coeff. Of friction (μ),
location of hinge pin (c), there are 3 different cases:
1. a > μ c (Desirable condition) Partially self
energising –P required for brake
2. a = μ c (Non-Desirable condition)
The break is self-locking. P=0
3. a < μ c (Non-Desirable condition) P= Negative
This is a dangerous operating condition, resulting in
uncontrolled braking and grabbing. The brake is
non-controllable by operator as he can not apply it.

Free Body Diagram (Anti-clockwise Rotation)
P (Braking effort) also depends upon the
direction of rotation of brake drum:
In design, the objective will be to
design for:
• Smaller braking effort
• Avoid dangerous and undesirable
braking conditions
The main disadvantage of block brake is
the tendency of brake drum to bend
under the action of normal force.
The remedy is to use two symmetrical
blocks at opposite sides of brake drum.

Fig. 12.5

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Chapter 12

Fig. Free Body Diagram of Forces

Chapter 12

Block Brake with Long Shoe
• The angle of contact in this case is > 450.
• For the short shoe brake normal Reaction (R) and frictional
forces are assumed to be concentrated at the center of shoe.
• This assumption is not applicable for brake with long shoe
brake.

Block brake with long shoe
Equation similar to Block
Brake with short shoe

Fig. 12.16 Internal Expanding Brake
Internal expanding brake
Application: Vehicle, conveyor, belt

Fig. 12.17 Free Body Diagram of Forces
Internal expanding Force diagrams

Chapter 12

Chapter 12
Torque

Chapter 12
Anticlockwise direction

Chapter 12
FORWARD AND REVERSE MOVEMENT

Chapter 12
Advantages

Chapter 12

Chapter 12

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Chapter 12

Chapter 12

Fig. 12.19

Band Brake
A simple band brake consists
of a flexible steel strip lined
with friction matrial, which
is pressed against the
braking drum.
When one end of band passes through the
fulcrum of the actuating lever, the brake
is called simple band brake.
The working of steel band is similar to that
of a flat belt operating at the zero
velocity

Free body diagram
Ratio of band tensions are given by:
P1= Tension in tight side of the band (N)
P2= Tension in slack side of the band (N)
μ = coeff of friction between friction lining
and brake drum
Θ = Anlgle of wrap (rad)
Mt = torque capacity of brake (Nmm)
R = radius of brake drum (mm)
Considering the forces acting on lever and
taking moments about the pivot:

Differential Band Brake (a) Construction (b) Free Body
Diagram
A band brake is called differential band brake when
neither end of band passes through the fulcrum of
actuating lever. Such brakes are designed for the
condition of self locking

Chapter 12

Advantages of self-locking: Although self-locking is
undesirable in speed control brakes. It is used to
advantage in Back-stop mechanism. A back-stop
brake is device, which is used to prevent the reverse
motion of drum would have harmful effects:
Applications:
• Bucket conveyors
• Hoisting application
• Material handling

Fig. 12.24

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Chapter 12

Chapter 12
Disk Brake
A disk brake is similar to a plate clutch except that one shaft is
replaced by a fixed member. Front wheel of motor cycle.
Vented disc brakes have a set of vanes, between the two sides of the
disc, that pumps air through the disc to provide cooling
Brake Pad

Chapter 12 Caliper Disk Brake
The disc brake is same like the brakes on a bicycle.
In a disc brake, the caliper which squeeze the brake pads the rotor instead of the
wheel, and the force is transmitted hydraulically instead of through a cable.
Friction between the pads and the disc slows the disc down.
Ex: lift trucks, farm machinery, front wheel of motor cycle
Single plate clutch equation is used for disk brakes.

Disk Brake Annular pad
The dimensions of annular pad are as follows:
Ro = Outer radius of pad (mm), Ri = Inner radius of pad (mm), θ= angular
dimension of pad (radians)
Since the area of pad is comparatively small, it is assumed that pressure on the
friction lining is uniform, The braking torque capacity according to uniform
pressure theory is given by:
Two shapes of pads
1. Annular
2. circular

Chapter 12 Disk Brake with Annular Pad

Disk Brake with Circular Pad
Friction radius of circular pad is given by:
Rf = δe
e = distance of pad centre from axis of disk (mm)
And values of δ are taken from table
R/e δ
0.0 1.0000
0.1 0.9833
0.2 0.9693
0.3 0.9572
0.4 0.9467
0.5 0.9375
R- radius of circular pad (mm)

Chapter 12
Thermal considerations
•The energy absorbed by the brake is converted into heat, which increases the
temperature of rubbing surfaces.
•As the temperature increases, the coefficient of friction decreases that adversely
affects the torque capacity of brake.
•At high temperature there is rapid wear of friction lining which reduces the life of the
lining
•The temperature rise should be kept within permissible limits.
•If it is assumed that all the heat generated during braking operation is absorbed by the
brake drum assembly, then:
Δt = E/(mc)

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Chapter 12